1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, using electrophotographic technique, and more particularly to an image forming apparatus having the function of erasing a charge on a non-image-exposing area of a photosensitive member to prevent a developing agent from adhering to the non-image-exposing area.
2. Description of Related Art
Conventional image forming apparatuses, such as a copying machine or a printer, using electrophotographic technique each are provided with such an image forming means as that shown in FIG. 7. Referring to FIG. 7, a photosensitive member 101 such as an organic photosensitive member is disposed adjacent to a conveying path H through which a sheet member S serving as a transfer member on which to form an image is conveyed (in FIG. 7, conveying rollers, etc., for conveying the sheet member S are omitted from illustration), so that the surface of the sheet member S being conveyed can be brought into close contact with the photosensitive member 101.
The photosensitive member 101 is charged uniformly to positive polarity or negative polarity by a charger 102, such as a corotron charger, a roller charger or the like. After that, an image exposure 103 is performed by an exposure means (not shown) to form an electrostatic latent image on the photosensitive member 101.
The electrostatic latent image is developed by a developing means 104 with a toner opposite in polarity to the electrostatic latent image, thereby becoming a toner image. Then, the toner image is transferred to the sheet member S (transfer member), such as paper, by supplying to the sheet member S a charge opposite in polarity to the toner.
The sheet member S having the toner image transferred thereto is then caused to pass through a fixing device 106, at which the toner image is heated and pressed to be fixed on the sheet member S.
On the other hand, any remaining part of toner which has been left on the photosensitive member 101 without being transferred is removed from the photosensitive member 101 by a cleaning means 107. Then, any residual charge on the photosensitive member 101, which has been cleaned, is removed by a pre-exposure lamp 108 or the like. After that, the photosensitive member 101 is again charged uniformly by the charger 102, so that an image forming cycle as described above is repeated.
Here, an area other than the image-exposing area of the photosensitive member 101 has a charge by the charger 102 remaining there and is, therefore, subjected to developing. Therefore, if the image-exposing area and the position and image-forming area of the sheet member S do not coincide with each other entirely, there is a possibility that a frame-like toner image is formed on a marginal part of the sheet member S .
In addition, in a case where reduction copying is performed or in a case where facing pages of a book or the like are copied, there is also a possibility that an unnecessary frame-like toner image is formed.
Such an unnecessary frame-like toner image not only lowers the image quality but also causes an increase in the amount of consumption of toner, scattering of toner, or the like. In particular, in a case where the frame-like toner image results in an image formed on the fore end of the sheet member S, there is a possibility that the sheet member S is wound onto the fixing roller and becomes difficult to separate from the fixing roller.
Therefore, in a conventional arrangement, in order to erase the above unnecessary frame-like image, there is provided an exposure means 109 for exposing a particular area (outside the image-exposing area) of the photosensitive member 101 (hereinafter, an exposure by the exposure means 109 being referred to as "frame-erasing exposure").
A fuse lamp or an LED (light-emitting diode) is used as a light source of the conventional exposure means 109 for performing the frame-erasing exposure. However, the fuse lamp has a large amount of he at generation, causing a large increase in temperature of the image forming apparatus body, so that the fuse lamp is not appropriate in a case where the image forming apparatus is arranged in a small size.
The LED has an amount of light emission gradually decreased due to the long-term usage thereof. Therefore, the initial amount of light emission of the LED is required to be set large. In this case, since the potential of an area of the photosensitive member 101 which is illuminated by the LED (frame-erasing exposing area) is lowered remarkably with respect to the potential of an area of the photosensitive member 101 which corresponds to the original (image-exposing area), such a potential difference as that shown in FIG. 8 occurs across the position of the end of the original. Therefore, there is a possibility that there occurs a phenomenon in which the frame-erasing exposing area is subjected to developing by the toner of the opposite polarity included in the developing agent (hereinafter, this phenomenon being referred to as "frame fogging").
In particular, in a case where the density of the background of the original is high (for example, newspaper), the above potential difference becomes more conspicuously large.
In the state in which the above potential difference is large, even if a developing electrode is used for developing, there is a possibility that, due to the peripheral electric-field effect, there occurs a phenomenon in which developing-agent particles are caused to adhere to a border part between the frame-erasing exposing area and the image-exposing area by a strong voltage contrast at the border part (hereinafter, this phenomenon being referred to as "border fogging").